Boris Johnson has sensationally been accused by Dominic Cummings of lacking “competence and integrity”, as the prime minister’s former most senior adviser lashed out after being accused of leaking by Downing Street.

In a post on his blog, Mr Cummings made the explosive allegation – denied by No 10 – that the prime minister had made a “mad” and “unethical” proposal to shut down a leak inquiry to avoid annoying his fiancee Carrie Symonds after one of her friends was implicated.

Mr Cummings, who resigned from 10 Downing Street in November amid escalating tensions within Mr Johnson’s febrile inner circle, said that the PM’s attempt to get Tory donors to pay for the redecoration of his flat was “possibly illegal”.

He claimed that Mr Johnson had authorised members of his team to brand him a leaker, despite knowing it to be untrue.

And he said he was ready to answer questions in full about his dealings with his former boss in a televised hearing of a parliamentary inquiry into the handling of the Covid pandemic on 26 May.

“It is sad to see the PM and his office fall so far below the standards of competence and integrity the country deserves,” wrote the former Vote Leave supremo.

But Mr Johnson played down the significance of the row, telling LBC radio: “I don’t think people give a monkey’s about this issue. What they care about is what were we doing to protect the health of the British public and that’s what I care about.”

Meanwhile, the prime minister was accused by the SNP leader in Westminster Ian Blackford of failing to honour a promise to parliament, after Downing Street released a statement summarising his contacts with inventor James Dyson, rather than the messages themselves.

Mr Blackford called for an independent public inquiry into “the Tory sleaze scandal and the accusations of privileged access, impropriety and possible illegality”, and wrote to cabinet secretary Simon Case urging him to secure Mr Johnson’s personal communications to ensure they are not deleted or tampered with.

Former Tory leader William Hague said that Mr Cummings was “clearly trying to do as much damage to the prime minister as possible”.

Lord Hague told Times Radio: “If all of these things were true, they would be damaging to anybody, including a prime minister.”

Mr Cummings’s devastating intervention was prompted by claims, briefed to sympathetic papers by a Downing Street source, that the PM suspected his “bitter” former adviser of being the source of a string of damaging leaks, including of text message exchanges with Dyson.

He flatly denied the allegation, offering to hand his phone over to Mr Case for examination as part of the leak inquiry launched on Thursday and challenging the PM to do the same.

And he said that Mr Case had already cleared him of the so-called “chatty rat” leak in October last year which forced the second Covid lockdown to be brought forward.

Mr Cummings dramatically claimed that Mr Johnson had considered ditching a probe into that leak after Mr Case said that evidence pointed towards the office of government adviser Henry Newman, a close friend of Ms Symonds.

“The PM was very upset about this,” wrote Mr Cummings. “He said to me afterwards, ‘If Newman is confirmed as the leaker then I will have to fire him, and this will cause me very serious problems with Carrie as they’re best friends … [pause] perhaps we could get the cabinet secretary to stop the leak inquiry?’

“I told him that this was ‘mad’ and totally unethical.”

Mr Cummings said that government officials would confirm under oath to any inquiry that he had encouraged Mr Case to continue with the probe.

And he said that he had warned Mr Johnson that his plan secretly to use donors’ money to pay for the reputedly six-figure cost of refurbishing the 11 Downing Street flat was “unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations”.

The government today announced that Mr Johnson had personally covered the cost of decorations beyond the £30,000 budgeted annually for maintenance from the public purse.

And Downing Street issued a statement to say that in relation to the flat, “at all times, the government and ministers have acted in accordance with the appropriate codes of conduct and electoral law”.

A spokesperson also insisted that the PM had “never interfered in a government leak inquiry”.

Labour deputy leader Angela Rayner said: “The government has spent the last 24 hours lurching between cover-ups and cock-ups.

“Labour is focusing on jobs, crime and the NHS, while the Conservatives are fighting each other like rats in a sack and slipping deeper and deeper into the mire of sleaze. 

“It shows breathtaking contempt for the country.”